(This is Part 6 of a 12 part series to provide you with some useful health and fitness info over the holiday season)

Metabolism. It’s a word we’ve all heard and especially in terms of talking about weight control. We usually use it as an excuse for why we can eat whatever we want when we are younger, and more often as an excuse for why we get bigger or can’t seem to meet our weight loss goals. The truth is, metabolism does have a lot to do with body weight and energy balance. Perhaps a better understanding of what metabolism is and how we can use it to work for us, instead of against us, is the secret to achieving and maintaining a fit, healthy body for running and for life.

Energy Expenditure

It is widely known that if you want to lose weight, you have to create a negative energy balance. In other words, you must expend more energy than you take in. Energy expenditure doesn’t just occur through physical activity. Our daily energy expenditure can actually be divided into three categories:

Basal (resting) metabolic rate (BMR)

Thermic effect of food (thermogenesis)

Energy expended through movement (DCE))

For this discussion, let’s look at the BMR. The basal, or resting metabolic rate is typically what people are referring to when they say their metabolism. It is defined as the amount of energy (or calories) required each day to keep your body functioning while at rest. More specifically, it is the energy that keeps the brain functioning, the heart beating, and the lungs breathing, in addition to many other cellular processes.

It is responsible for about 60-75% of our daily energy expenditure, but may account for less in individuals who are very physically active.

Determining the BMR

BMR can vary greatly between individuals and there are a few personal characteristics that determine one’s metabolism. The first is body size. In general, larger people have higher metabolic rates than smaller people. It is based on surface area so that a greater surface area equates to a higher metabolic rate. Therefore if a tall person and a short person both weighed the same, the taller person would have a higher metabolic rate due to the larger surface area. However, the composition of body weight is the biggest determinant of metabolic rate. Fat-free mass, or FFM (muscle, bones, organs) is metabolically active (calorie-burning) tissue so the more you have, the higher your metabolism, hence the importance of incorporating resistance training. If two individuals were the same height and weight, the one with more FFM would have the higher BMR. In general, athletes have RMRs that are ~5% higher than their non-athletic counterparts due to more muscle mass as opposed to fat mass. Knowing your FFM is the best way of determining your BMR and therefore your daily caloric needs. The preferred method of obtaining FFM is through underwater weighing, though not always feasible or practical. Other options such as body fat measurements through calipers or body fat scales can provide a reasonable estimate. Once FFM is determined it can be used in a prediction equation, like the Cunningham equation, to determine BMR:

BMR = 370 + (21.6 x FFM[kg])

Other Factors on Metabolism

Age and sex also have an effect on metabolic rate. One’s metabolism is the highest during periods of rapid growth such as infancy or puberty, which is why feeding babies and ravenous teenagers is important. As we age, however, we start to lose muscle mass and thus the metabolism begins to slow. It is estimated that we lose ~2-3% of our previous BMR for each decade of life past 30 years old. Also, since women generally have more body fat and less muscle than men, men typically have higher metabolic rates but are still subject to declining BMR with age. Other factors to consider when thinking about metabolism include:

Hormonal disorders such as hyperthyroidism, which will increase your BMR, and hypothyroidism, which will decrease your BMR.

Having a fever increases the metabolic rate by ~7% for every degree increase above 98.6° F.

Finally, living and exercising in tropical climates can increase BMR anywhere from 5-20%.

Metabolism’s Affect on Weight Loss

The big question is how to manage weight loss and metabolism to find a healthy weight that will allow you to perform optimally, but is also easy to maintain. One of those tips was to make sure you don’t cut calories too drastically. Having too few calories can lead to the body breaking down protein, and therefore muscle mass, for energy. As we just learned muscle mass is the biggest determinant of metabolic rate and the less of it we have, the lower our metabolism will be. The lower the metabolism, the less calories are needed for daily maintenance, and the harder it becomes to lose weight. Another issue is that the more we restrict our calories, the more efficient the body becomes at using the calories that we do give it. Normally efficiency is a good thing, unless we are trying to lose weight. When trying to lose weight, or create a negative energy balance, we don’t want the body to be efficient at using calories so that it has to work harder and thus burn more calories. The final difficulty involving weight loss and metabolism is that as we lose weight, we require less energy (because BMR is determined mainly by body mass). This means you need to continually decrease your intake to account for the decrease in metabolic rate.

Managing Weight Loss and Metabolism

To off-set the natural decline in metabolism that comes with age, start and continue a weight training program and do it 2-3 days per week to preserve lean muscle mass. To avoid big drops in BMR, limit calorie restriction to ~15% less than what is required to meet your maintenance and training needs. So if you needed 2300 calories a day to meet your BMR + training expenditures, you should only reduce that by ~345 calories per day (consume ~ 1955 calories per day). Be realistic about your weight loss goals and once you reach those goals, stop dieting. Your BMR will return to normal once calorie restriction has ceased and a normal caloric intake is resumed. Don’t fight against your metabolism but learn how to structure your nutrition to fit with it. Some factors contributing to metabolism are out of our control or very difficult to change. Furthermore, metabolism is a finely tuned and highly regulated operation of our bodies and we function best when it is in balance.

Jeff Harrison is a fitness coach based in Pottstown, PA. He received a BS in Exercise and Sport Science from Penn State University and is an NSCA Certified Strength and Conditioning Specialist (CSCS), NSCA Certified Personal Trainer (NSCA-CPT) and
ACE Advanced Health and Fitness Specialist (ACE-AHFS). Jeff's articles have been published in peer-reviewed journals as well as consumer oriented websites and magazines.